I've read many post-apocalyptic stories, but this is the first pre-apocalyptic book I've encountered! And its a good un.
Written in 1987 and set in the mid 90s, after Jupiter's moon Europa disappears a strange object appears in the American desert resembling a cinder cone, and next to it is found a strange dying alien, The alien speaks English and has a message for Earth: "I'm afraid I have bad news".
Later a second cone is found in Australia but this time mechanical beings, robots, appear around it but these seem to be benevolent. While in the US the scientists are getting worried, those in Australia believe the aliens are our allies. Later, people encounter strange small robots; at first they fear them but it seems these 'spiders' are recruiting certain people on Earth for some purpose. These 'possessed' people receive messages from a network direct to their consciousness- a kind of direct internet.
Meanwhile a cult of 'Forge of Godders' springs up, setting up vigils next to the cinder cones, rather like druids attending Stonehenge.
No-one is really sure of the aliens intent until it is too late...
This is hard SF but its very reader friendly; there's no scientific preaching or technobabble, and despite being over 450 pages the story flows really well. So much so I'd have to say this is the best Greg Bear book I've read so far!